The ASM-1 LFOs are basically low-frequency function generators that produce two waveforms.
The basic oscillator circuit consists of the two op-amps U501b and U501a connected respectively as an integrator and a Schmitt trigger. When the output of U1a is positive the full positive volatge of U1a is applied to P501, so a current flows into the integrator through R504. The output of U501b ramps negative until it reaches about -5V, when the voltage on the non-inverting input of U501a will fall below the voltage on the inverting input (zero volts) and the output of U501a will swing negative. The full negative output voltage of U501a is applied to P501. Current will flow out of the integrator through R504, and the integrator output will ramp positive until it reaches about +5V, when the voltage on the non-inverting input of U501a will rise above zero and the output of U501a will swing positive. The whole cycle then repeats.
The output from U501b is thus a triangular waveform with a peak-to-peak voltage of about 10V, while at the output of U501a a squarewave of the same frequency is available.
P501 is used to adjust the frequency of the LFO by varying the voltage applied to the integrator input, which alters the integrator input current and hence the rate at which the integrator ramps positive or negative.
The value of R503 determines the minimum integrator input voltage that can be set by P501, and hence the minimum frequency of the LFO. The value of R3 must not be chosen too high or the minimum frequency of the LFO will be too great. On the other hand it should not be chosen too low, or the minimum integrator input current at the minimum setting of P501 will be comparable with the input currents of U501b. This will result in unreliable operation of the oscillator at low frequencies. The optimum value for R503 is 22R, but the user is free to experiment with this for best results.
The
squarewave at the output of U501a has an amplitude of +/- 15 volts,
so the resistor attenuator network R505/R506 is included in the output
path to provide the standard +/- 5 volt level.